It is very possible that it could be a food allergy... Beneful is actually one of the worst dog foods out there, lots of corn fillers and not much meat which can cause allergies like what your seeing the chewing and scratching.
I'd recommend trying him on a different higher quality food though I'd make sure to switch him slowly to a new food(i.e starting off by mixing in a little of the new food with his and slowly adding more). A higher quality food is going to be more expensive BUT with it being more nutritionally dense you do not have to feed as much and you will have less waste.
I'd probably try some sort of fish formula like taste of the wild pacific stream, Acana Pacifica, Fromm salmon, earthborn holistic coastal catch to name a few... I'm sure others will have more suggestions.
Another thing is you can go to dogfoodadvisory.com... It's not the end all be all but you can get an idea of the quality of different foods. Here is beneful for example, http://www.dogfoodadvisor.com/dog-fo...-dog-food-dry/